This application claims the priority benefit of Taiwan application serial no. 90132,742, filed Dec. 28, 2001.
1. Field of Invention
The present invention relates to a method for controlling the formation of an intermetallic compound in solder joints. More particularly, the present invention relates to a method for controlling the types of the intermetallic compounds formed between the SnAgCu solders and the Ni-bearing substrate by adjusting the copper concentration in the SnAgCu solder.
2. Description of Related Art
Integrated circuitry (IC) packaging is an important part of the semiconductor fabrication backend processes, which protects each chip and electrically connects the pads on the chips to the printed circuit board. The packages of the chip and the printed circuit board are connected through solders, while the soldering pads in contact with the solders commonly use Au/Ni or Pd/Ni bi-layer structure as a surface finish or a metallization. During the reflow soldering process, gold or palladium in the pad first reacts with the solder and was then merged into the solder. After using up gold or palladium, the underlying nickel keeps reacting with the solder to form soldering intermetallic compounds. If the mechanical strength of a solder joint is not strong enough, it may cause reliability problems.
The Pbxe2x80x94Sn solder is one of the most widely used materials for soldering. However, lead containing in the Pbxe2x80x94Sn solder, especially its oxides, can be toxic and harmful to the environment and human health. Therefore, lead-bearing solders are proposed to be banned in many countries.
To replace the lead-bearing solders, SnAgCu series of solders are generally considered to be one of the most promising replacements. The SnAgCu solders with different compositions have different physical and electrical properties. The prior art researches relating to SnAgCu solder series include U.S. Pat. No. 5,527,628 and Japanese Patent Application No. 2001-504760. The prior art researches mostly focus on the properties of SnAgCu solder series, rather than on the reactions between solders and pads or the soldering technique for connecting both. None of the prior art materials disclose a method for controlling the types of the intermetallic compounds between the SnAgCu solders and the Ni-bearing substrate by adjusting the compositions of the SnAgCu solders.
Accordingly, the invention provides a method for controlling the types of the intermetallic compounds between the SnAgCu solders and the Ni-bearing substrate by adjusting the compositions of the SnAgCu solders.
The present invention relates to a method for controlling the formation of the intermetallic compounds. The types of the intermetallic compounds between the SnAgCu solders and the Ni-bearing substrate can be controlled by adjusting the copper concentration in the SnAgCu solders.
As embodied and broadly described herein, the invention provides a method for controlling the formation of the intermetallic compounds. A substrate having a Ni-bearing bonding pad, for example, Au/Ni double-layered pad or Pd/Ni double-layered pad, is provided. An amount of SnAgCu solder is formed on the Ni-bearing pad. A reflow soldering process is performed, so that the SnAgCu solder reacts with the Ni-bearing pad to form a solder joint. If the copper concentration in SnAgCu solder is between 0.6 wt. % to 3.0 wt. % (including 0.6 wt. %), the continuous soldering intermetallic interlayer formed at the interface is a (Cu1xe2x88x92xNix)6Sn5 layer. If the copper concentration in SnAgCu solder is between 0 wt. % to 0.4 wt. % (including 0.4 wt. %), the continuous soldering interlayer is a (Ni1xe2x88x92yCuy)3Sn4 layer. If the copper concentration in the SnAgCu solder is between 0.4 wt. % to 0.6 wt. %, the soldering interlayer is a composite layer including the continuous (Cu1xe2x88x92xNix)6Sn5 layer and the continuous (Ni1xe2x88x92yCuy)3Sn4 layer.
The present invention relates to a method for controlling the formation of the intermetallic compounds. The types of the intermetallic compounds can be controlled by adjusting the minor copper concentration in the SnAgCu solders.
The method of the present invention for controlling the formation of the intermetallic compounds can not only control the types of the intermetallic compounds, but also growth rates of the soldering intermetallic compounds.
The method of the present invention for controlling the formation of the intermetallic compounds is greatly influenced by the copper concentration in the solder. Particularly, if the copper concentration is between 0.4 wt. % to 0.6 wt. %, the types of the intermetallic compounds can vary significantly due to minor processing errors in Cu concentration, further deteriorating reliability of the solder. Therefore, the present invention provides a method for controlling the formation of desirable intermetallic compounds at the interface.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.